To improve the corrosion resistance, metal coatings are applied to sheet or strip steel which in most applications are based on zinc or zinc alloys. Such zinc or zinc alloy coatings, because of their barrier and cathodic protective effect, provide good corrosion protection in practical use for the correspondingly coated sheet steel.
The thickness of the coating required for adequate corrosion resistance in the prior art however causes problems in processing i.e. when forming and welding. This applies for example when in practical use flanges subject to high corrosion load are to be spot-welded. This requirement exists in particular in the field of car body construction, in general building applications or in the construction of housings for domestic appliances. The connection produced by such welding, for an adequate welding current, must have a minimum spot diameter of 4×√{square root over (t)}(t=individual sheet thickness) and be able to be welded without spatter.
In the context of the problems in processing conventional sheets coated with a relatively thick Zn layer, highly corrosion-resistant Zn—Mg or Zn—Mg—Al layer systems have been developed which, with a greatly reduced layer thickness, offer corrosion protection comparable to that of a conventional 7.5 μm thick zinc coating but are significantly easier to process.
One possibility for producing such hot galvanised sheet steel with increased corrosion resistance and simultaneously reduced coating mass is described in EP 0 038 904 B1. A zinc coating containing 0.2 wt. % Al and 0.5 wt. % Mg is applied to a steel substrate by hot dip coating. Although the metal coated in this way has improved resistance to rust formation, in practice it does not fulfil the requirements imposed today for corrosion resistance of such panels, in particular in the area of connecting flanges of a car body.
A further sheet provided with a metallic protective coating with increased corrosion resistance is known from EP 1 621 645 A1. The sheet steel described there is coated, by conventional hot galvanising, with a protective coating which contains (in wt. %) 0.3 to 2.3% Mg, 0.6 to 2.3% Al, optionally <0.2% other active constituents and the remainder zinc and unavoidable impurities. Due to the high proportion of Al and Mg, such metal has particularly good resistance to corrosion. Practical tests however have shown that even the panels produced according to EP 1 621 645 A1 do not fulfil the requirements imposed by the processing industry for the weldability of such panels. It is also shown that the panels concerned have a phosphatisation capacity which is inadequate according to present standards.